1. Field of the Invention
The present invention relates to a plane carbon commutator used as a commutator for a motor of a fuel pump and the like, and to a producing method of the plane carbon commutator. More particularly, the present invention relates to a plane carbon commutator fore reliably connect segment and carbon in the commutator, and to a producing method of the plane carbon commutator.
2. Description of the Related Art
A plane carbon commutator comprises a metal segment attached to an end face of a commutator body made of mold resin, and carbon attached to the segment. As a producing method of this kind of plane carbon commutator, there are the following methods (A) to (D) for example.
(A) When carbon is formed, a base metal which is a segment is inserted into the carbon and the base metal and the carbon are integrally formed and burnt and then, the base metal which was integrally molded with the carbon is integrally molded with the mold resin, thereby forming an insulator portion (see Japanese Patent Application Laid-open No. H7-264812 for example).
(B) An insulator and a metal base are previously integrally formed by integrally molding or the like and then, carbon is attached to a face of the metal base by soldering or conductive adhesive (see Japanese Patent Application Laid-open No. H5-502974 for example).
(C) An insulator and a metal base are previously integrally formed by integrally molding or the like and then, carbon is formed on a face of the metal base and burnt (see Japanese Utility Model Publication No. H7-42223 for example).
(D) An insulator and a metal base are previously prepared separately, and when carbon is formed, the metal base and the insulator are integrally molded with the carbon (see Japanese Patent Application Laid-open No. H6-178503 for example).
In the method (A), a temperature for burning the carbon is high as higher as about 600° C. or higher. Therefore, the integrally formed metal base is softened, and the product has a problem in terms of precision and strength. Thereupon, the carbon can be burnt at a low temperature about 200° C., but in such a case, the quality of material of the carbon itself becomes special, and there is a problem in various characteristics such as hardness, electrical resistance and gasoline resistance.
In the method (B), the carbon can be previously burnt singly, and there is no problem in the quality of material of the carbon itself. However, if the carbon is soldered to the face of the metal base, there is an adverse possibility that the solder is loosened by a high temperature of wire at the time of assembling of a motor.
If the metal base and the carbon are adhered by the conductive adhesive, this structure requires an adhesive having both conductive property and gasoline resistance, which is expensive. Further, even if the adhesive has the conductive property, electrical resistance thereof is greater as compared with the carbon and the metal base, and there is a problem that this portion is prone to generate heat and a material thereof is prone to be changed when a motor is driven.
In the method (C), the insulator made of resin is carbonized by a high temperature when the carbon is burnt on the face of the metal base. Therefore, the carbon must be burnt at a low temperature, and there is a problem in the quality of material of the carbon.
The method (D) has the same problem as that of the method (C).
Thereupon, in order to solve the above-described conventional problems, the present assignee filed Japanese Patent Application No. H9-51991 (Japanese Patent Application Laid-open No. 10-4653, which will be referred to as “prior example” hereinafter). In the prior example, a segment is formed with an engaging hole, and an engaging projection provided with carbon that was previously burnt at a high temperature is engaged into the engaging hole and integrally formed as one piece. Therefore, the segment and the carbon can be integrally formed with out using solder or adhesive, and the initial object could be achieved.
In the prior example, when the engaging projection formed on the carbon is engage with the engaging hole formed on the segment and attached and integrally formed, shrinkage fit and press-fit are carried out, and after the engaging projection is engaged with and inserted into the engaging hole, burring or the like is carried out. The engaging projection is fastened by a projection formed by the burring.
Therefore, when the engaging projection of the carbon is inserted into the engaging hold of the segment and both of them are integrally formed, there is a problem that the process for integrally forming them is troublesome.